Electrodes incorporating composites of graphene and selenium-sulfur compounds for improved rechargeable lithium batteries

ABSTRACT

Embodiments of the present invention relate to a method to enable fabrication of a battery electrode comprising forming a conductive compound comprising a selenium-sulfur compound and a conductive additive. The conductive compound is applied onto a conductive substrate utilizing one or more of casting, pressing, ink jet printing, and screen printing. The selenium-sulfur compound is present as Se x S 8-x  and 1&lt;x&lt;8. The conductive additive comprises individual graphene sheets.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/055,505 filed Feb. 26, 2016, which claims priority to U.S.Provisional Application No. 62/121,330 filed Feb. 26, 2015, each ofwhich are hereby incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Grant No.DE-AR0000319 awarded by the Advanced Research Projects Agency-Energy(ARPA-E) of the United States Department of Energy. The U.S. Governmenthas certain rights in this invention.

BACKGROUND Technical Field

The present invention relates generally to battery electrodes andspecifically to battery electrodes incorporating composites of grapheneand selenium-sulfur compounds. Lithium sulfur batteries have a hightheoretical capacity for storing energy. However, sulfur is anelectrical insulator, which typically requires combining the materialwith a conductive additive, such as a carbonaceous material, to form acomposite electrode. While such a material may increase conductivity,the carbonaceous material does not contribute to the capacity of thebatter to store charge. Furthermore, the carbonaceous material typicallyincreases the mass of the device, which concomitantly reduces theapparent charge density of the device.

The charge and discharge of the device at high rates typically requiresthat a significant mass fraction of the electrode comprise theconductive additive, which can result in a deleterious effect on theapparent charge capacity density of the device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a graph reflecting capacity at various rates of batteriescomprising differing Se—S compounds, in accordance with an embodiment ofthe present invention.

FIG. 2 depicts a graph reflecting discharge profiles at C/10 ofbatteries comprising differing Se—S compounds, in accordance with anembodiment of the present invention.

FIG. 3 depicts a graph reflecting discharge profiles at 1 C of batteriescomprising differing Se—S compounds, in accordance with an embodiment ofthe present invention.

DETAILED DESCRIPTION

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration but are not intended tobe exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

Lithium sulfur (“Li—S”) batteries have a high theoretical capacity forstoring energy. However, sulfur is an electrical insulator, which istypically requires combining the material with a conductive additive,such as a carbonaceous material, to form a composite electrode. Whilesuch material may increase conductivity, the carbonaceous material doesnot contribute to the capacity of the battery to store charge.Furthermore, the carbonaceous material typically increases the mass ofthe device, which concomitantly reduces the apparent charge density ofthe device. The charge and discharge of the device at high ratestypically requires that a significant mass fraction of the electrodecomprise the conductive additive, which can result in a deleteriouseffect on the apparent charge capacity density of the device.

The development of high-energy high density electrodes for Li-ionbatteries can facilitate the development and adaptation of electricaldevices, such as consumer electronics and electric vehicles, andrenewable energy storage. Embodiments of the present invention compriseelectrodes incorporating composites of individual graphene sheets andselenium-sulfur compounds for improving electrical characteristics ofbatteries, such as lithium batteries. Li—S batteries typically exhibitpoor cycle life due to the solubility of intermediate species formed asS converts to Li₂S during charge and discharge of the battery. Theseintermediates are referred to as polysulfides. During charge ordischarge, the dissolved polysulfides migrate from the cathode to theanode where they can precipitate and foul the anode leading toirreversible loss of battery capacity during cycling.

Replacing all of part of the sulfur material with a more conductivematerial can reduce the quantity of carbonaceous material needed.Selenium (“Se”) does not have as high a theoretical capacity as sulfur(1675 mAh/g for sulfur versus 680 mAh/g for Se), but does have a higherconductivity. Hence, the addition of Se to electrode material of thepresent invention can theoretically increase the conductance of theelectrode, but at the cost of lowering capacity.

Surprisingly, electrodes of the present invention comprises acomposition of Se, sulfur, and individual graphene sheets (“thecomposition”) that does not exhibit a decrease in capacity compared toelectrodes formed of a composition of graphene sheets and sulfur alone.On the contrary, the capacity of such composites actually exceed thecapacity of the Se deficient electrode when operated at higher rates. Inaddition, the composition displayed an improved cycle-life compared towhen the composition lacks Se, which suggests that the presence of Se inthe composition addresses the aforementioned issues exhibited with thepolysulfides.

The Se—S compounds of the present invention can b utilized as batteryelectrodes, such as those having high mass and volume specificelectrical energy storage capacity. The Se—S compounds of the presentinvention are limited to Se_(x)S_(8-x), wherein x is used to indicatethe stoichiometry of the compound, and wherein 0<x<8. Hence, x alsoencompasses non-integer values. In certain embodiments 1≤x≤4 exhibitpreferred electrical characteristics.

Se—S compounds can also comprise conductive additives to furtherincrease the electrode's performance. Conductive additives can comprisecarbonaceous material, such as graphite, multi-layered graphene, singlesheets of graphene, thermally exfoliated graphite oxide, chemically orthermally reduces graphene oxide or graphite oxide, single- andmulti-walled carbon nanotubes, hard carbon, soft carbon, carbonaerogels, carbon xerogels, carbide-derived carbons, templated carbons oractivated carbons. Conductive additives can comprise materials capableof conducting electrons or holes such as a semiconductor, semi-metals,and metals. Note that mesoporous carbon is not compatible withembodiments of the present invention as the desired performanceimprovements are lacking.

Conductive additives can be present in the final battery electrode at aloading of 0 to 10%, 10% to 20%, 20% to 30%, 30% to 40%, 40% to 50%, 50%to 60%, 60% to 70%, 70% to 80%, 80% to 90%, and/or 90% to 99% on a drymass basis. Se and S can be mixed together in various ways, such assimple mixing, milling/grinding (wet or dry), ultrasonication,dissolution in solvent and precipitation, melting, sublimation or vapordeposition. In particular, dissolution of sulfur and Se can be achievedat various temperatures in solvents, such tetrahydrofuran, carbondisulfide, dimethylsulfoxide or other solvents capable of dissolvingsignificant amounts of these materials.

Se and S can be used in a dissolved form or precipitated to form a solidcompound by a suitable method such as cooling from solution to reducethe Se and S solubility or adding the solution to another solvent, suchas water, to reduce the solubility thereof and cause precipitation. Theconductive additive can be mixed with the Se and S prior to orsubsequent to the formation of the Se—S compound. For example, Se and Scan be blended with the conductive additive by grinding/milling, etc. Seand S can be melted and mixed directly with the conductive additive. Seand S can be sublimated or vaporized and condensed directly on thesurface of the conductive additive. Dissolved Se and S can also beprecipitated from solution directly onto the conductive additive fromone or more of the solvents disclosed herein. Alternatively, Se and/or Scan be formed onto the conductive additive by electrodeposition.

The crystalline phrase, morphology, or nanostructure of the resultingSe—S compound can be tuned using various techniques implemented before,during, or after mixing with the conductive additive. For example, astructure directing agent such as a surfactant, lipid emulsion eitherbound to the additive or dissolved or dispersed in solvent that can beused to template the growth of Se—S crystals or amorphous particles witha predefined shape or size. A conductive additive with a controlledpore-size or structure can also be used to control the size and shape ofthe deposited Se—S compound.

The resulting materials can be deposited onto a conductive currentcollector, which includes, but is not limited to, a conductive carbon,aluminum, aluminum coated with a conductive carbon material, copper,titanium, or tungsten. The carbon or metallic coating can be a thin filmlaminated on a plastic substrate, such as polypropylene or polyethyleneterephthalate. The current collector can be in the form of a sheet ormesh. The Se—S compound and/or carbon additive which form the batteryelectrode can be coated on or impregnated into the current collectorusing a suitable technique, which includes, but is not limited to,casting, pressing, ink jet printing, and screen printing.

Electrode fabrication may involve the use of a binder such as asulfonated tetrafluoroethylene based fluoropolymer-copolymer,polyvinylidene fluoride, and polytetrafluorethylene. Such binders may beadded during or subsequent formation of the Se—S compound and mixed withthe conductive additive. Battery operation requires an appropriateelectrolyte consisting of a solvent and dissolved salt or a molten saltalone is typically utilized to facilitate Li ion transport between theSe—S compound and a second electrode. The solvent can comprise water,1,3-dioxolane, dimethylether, ethylene carbonate, propylene carbonate,diethylene carbonate, γ-butyrolactone and/or a solvent capable ofdissolving lithium salts.

The solvent can comprise molten salts, such as 1-ethyl3-methylimidazolium bis(trifluoromethylsulfonyl) imide. Lithium saltscan comprise lithium perchlorate, lithium bis(trifluoromethylsulfonyl)imide, lithium nitrate, lithium tetrafluoroborate, and/or lithiumhexafluorophosphate. The electrolyte can be added to the electrodematerial before or after coating onto the current collector. A suitablemembrane can be utilized to electrically separate the Se—S compoundcontaining electrode from the second electrode. The membrane cancomprise a porous polymer that will not dissolve in the electrolyte,such as polypropylene, polyethylene terephthalate, cellulose, andpolytetrafluoroethylene.

Battery operation requires that the electrode comprising the compositionbe coupled to a second electrode. For example, the second electrode maycomprise lithium metal which is in ionic but not electroniccommunication with the composition. Alternatively, the second electrodecan be a material which can reversibly accept or emit lithium ions andhas a resting potential above or below that of the Se—S compound, such agraphite, lithiated graphite or silicon. Batteries of the presentinvention may be assembled in typical battery configurations, whichincludes, but is not limited to, coin cells and pouch cells.

As will be understood by one skilled in the art, for any and allpurposes, particularly in terms of providing a written description, allranges disclosed herein also encompass any and all possible subrangesand combinations of subranges thereof. Any listed range can be easilyrecognized as sufficiently describing and enabling the same range beingbroken down into at least equal halves, thirds, quarters, fifths,tenths, etc. As a non-limiting example, each range discussed herein canbe readily broken down into a lower third, middle third and upper third,etc. As will also be understood by one skilled in the art all languagesuch as “up to,” “at least,” “greater than,” “less than,” and the likeinclude the number recited and refer to ranges which can be subsequentlybroken down into subranges as discussed above. Finally, as will beunderstood by one skilled in the art, a range includes each individualmember.

The present invention, thus generally described, will be understood morereadily by reference to the following examples, which are provided byway of illustration and are not intended to be limiting of the presentinvention.

EXAMPLES

The disclosed approach has been tested experimentally. Experimental datais reflected in FIGS. 1-3. Here, two different Se—S compounds, SeS₇ andSe₂S₆, were prepared by dissolving Se and S in an appropriate ratio indimethylsulfoxide. Thermally exfoliated graphite oxide, as disclosed inU.S. Pat. No. 7,658,901 to Prud'Homme et al., which is incorporatedherein in its entirety, was used as the carbonaceous material anddispersed in water using a surfactant. The two solutions were combined,which resulted in the precipitation of the Se—S compound onto thecarbonaceous material. Subsequent to filtration and washing, the filtercake was pressed into an aluminum mesh, dried and tested as a batterysandwiching a porous polypropylene membrane between this electrode andlithium metal using a standard stainless steel coin cell assembly.

The current used to discharge the cells is specified in terms of aC-rate, wherein C is 1675 mAh/g. The loading of each electrode was about4.5 mg/cm². The same procedure was used to prepare electrodes lackingSe. A comparison of the behavior at various C-rates is depicted inFIG. 1. Here, both samples comprising SeS₇ and Se₂S₆ exceed the capacityof samples comprising S at low charge/discharge rates and exhibit animproved capacity of about 300% and about 350%, respectively, at 1 C.Hence, samples comprising SeS₇ and Se₂S₆ both exhibit an improvedcycling stability.

FIGS. 2-3 compare the capacity versus discharge curves for theaforementioned samples at two rates of charge/discharge. FIG. 2 comparesdischarge capacities of the aforementioned samples, in accordance withan embodiment of the present invention. Here, FIG. 2 illustrates thatthe addition of Se does not statistically reduce the discharge capacityof the battery compared to samples lacking Se. FIG. 3 depicts thedischarge curves at a fast discharge rate of 1 C, in accordance with anembodiment of the present invention. Here, the S only battery exhibiteda discharge capacity of about 225 mAh/g. In contrast, the dischargecapacity of the battery samples comprising SeS₇ and Se₂S₆ ranged fromabout 590 mAh/g to about 675 mAh/g at 1 C.

While certain embodiments have been illustrated and described, it shouldbe understood that changes and modifications can be made therein inaccordance with ordinary skill in the art without departing from thetechnology in its broader aspects as defined in the following claims. Asused herein, “about” will be understood by persons of ordinary skill inthe art and will vary to some extent depending upon the context in whichit is used. If there are uses of the term which are not clear to personsof ordinary skill in the art, given the context in which it is used,“about” will mean up to plus or minus 2% of the particular term.

The embodiments, illustratively described herein may suitably bepracticed in the absence of any element or elements, limitation orlimitations, not specifically disclosed herein. Thus, for example, theterms “comprising,” “including,” “containing,” etc. shall be readexpansively and without limitation. Additionally, the terms andexpressions employed herein have been used as terms of description andnot of limitation, and there is no intention in the use of such termsand expressions of excluding any equivalents of the features shown anddescribed or portions thereof, but it is recognized that variousmodifications are possible within the scope of the claimed technology.Additionally, the phrase “consisting essentially of” will be understoodto include those elements specifically recited and those additionalelements that do not materially affect the basic and novelcharacteristics of the claimed technology. The phrase “consisting of”excludes any element not specified.

The present disclosure is not to be limited in terms of the particularembodiments described in this application. Many modifications andvariations can be made without departing from its spirit and scope, aswill be apparent to those skilled in the art. Functionally equivalentmethods and compositions within the scope of the disclosure, in additionto those enumerated herein, will be apparent to those skilled in the artfrom the foregoing descriptions. Such modifications and variations areintended to fall within the scope of the appended claims. The presentdisclosure is to be limited only by the terms of the appended claims,along with the full scope of equivalents to which such claims areentitled. It is to be understood that this disclosure is not limited toparticular methods, reagents, compounds compositions or biologicalsystems, which can of course vary. It is also to be understood that theterminology used herein is for the purpose of describing particularembodiments only, and is not intended to be limiting.

What is claimed is:
 1. A method to enable fabrication of a batteryelectrode comprising: forming a conductive compound comprising aselenium-sulfur compound and a conductive additive; applying theconductive compound onto a conductive substrate utilizing one or more ofcasting, pressing, ink jet printing, and screen printing; and whereinthe selenium-sulfur compound is present as Se_(x)S_(8-x); 1<x<8; and theconductive additive comprises individual graphene sheets.
 2. The methodof claim 1, wherein 1<x≤4.
 3. The method of claim 1, further comprising:coating the conductive substrate with a conductive carbon material,copper, titanium, and/or tungsten, and wherein the conductive substratecomprises aluminum; and wherein the conductive substrate is in the formof a sheet or mesh.
 4. The method of claim 1, wherein forming theselenium-sulfur compound is formed by: one or more of mixing, wet or drymilling, wet or dry grinding, ultrasonication, and dissolution insolvent; and one or more of precipitation, melting, sublimation, andvapor deposition.
 5. The method of claim 1, wherein forming theconductive composition comprises: mixing a binder with theselenium-sulfur compound and the conductive additive; and wherein thebinder comprises one or more of a sulfonated tetrafluoroethylene basedfluoropolymer-copolymer; polyvinylidene fluoride, andpolytetrafluorethylene.
 6. The method of claim 1, further comprising:laminating the conductive substrate with one or more of a carbonaceousmaterial and a metallic coating thin film; wherein the conductivesubstrate is in the form of a sheet or mesh; and comprises a polymersubstrate.
 7. The method of claim 1, wherein forming the Se—S compoundcomprises sublimating or vaporizing and condensing the Se—S compound onto a surface of the conductive additive.
 8. The method of claim 1,wherein forming the conductive compound comprises utilizing acarbonaceous material comprising one or more of graphite, graphiteoxide, carbon nanotubes, a hard carbon, a soft carbon, a carbon aerogel,a carbon xerogel, carbide-derived carbon, a templated carbon, andactivated carbon.